For example, where an outdoor unit of an air conditioner is mounted to a concrete wall, the concrete wall is first bored, anchor bolts are mounted into the thus bored holes, and the outdoor unit is fixed to the anchor bolts with screws.
As a boring tool for making the above-described bored holes, there is known a hammer drill and a diamond drill. The hammer drill is a tool for hammering and boring the concrete wall, while a carbide tip of a bit is pierced to break the wall. However, this tool causes troublesome hammering noise.
On the other hand, the diamond drill is a tool in which a diamond grindstone body is fixed to the tip of a base constituting a seat, making a hole by firmly pressing a rotating bit against the concrete wall and cutting the surface thereof. Therefore, this tool is advantageous in lower working noise.
Incidentally, the diamond drill must continue to constantly press a bit attached to the tip thereof, thereby generating high heat resulting from frictional heat. Thus, there is generally used a wet-type diamond drill which allows water to flow inside the bit (refer to Patent Documents 1 and 2). However, since this type requires a hole made for allowing water to pass through a center thereof, the bit must be hollow. Therefore, in the case of a core drill bit, it requires, a device for circulating cooling water and also requires means for removing concrete debris (swarf) remaining inside the hollow bit.
In contrast, a dry-type diamond drill, which is free of water, is constituted so that it is at least partially solid, the tip thereof is formed in a flat shape and a diamond grindstone body is fixed on an iron-based seat. The dry-type diamond drill does not require a device for circulating water or means for removing concrete debris. This drill is provided with a slit-like recessed cutout portion opened to one side, and swarf is to be removed outside from the recessed cutout portion.    Patent Document 1: JP-Y-05-030891    Patent Document 2: JP-A-05-245827
However, a conventional non-core drill bit (hereinafter, simply referred to as a bit) has the following disadvantages.    (1) In FIG. 9(a), since a bit 20 has a solid base 23 (the numeral 22 denotes a recessed cutout portion of a diamond grindstone body 21), cutting is done by the outer periphery side and the center side of the diamond grindstone body 21. In this case, there is a difference in rotating speed between the outer periphery side 21a and the center side 21b. When the bit 20 is rotated to perform boring, the outer periphery side 21a is higher in speed, thereby responding to concrete more efficiently. However, as the center side 21b is slower in speed, it responds to concrete less efficiently. Since the boring speed is dependent on working capability of the center side which is lower in working capability, the boring speed is slow while the bit 20 is new. After repetition of boring work, the bit 20 wears at the center, by which hole-cutting can be performed on the outer periphery side to improve the performance. It takes, however, some time by the time a predetermined performance is obtained.    (2) The performance will be improved as a bit wears at the center. However, due to a difference in rotating speed between the outer periphery side and the inner periphery side of the bit, a conically projected portion uncut and remaining develops at the center of the leading end of a hole 15. At the beginning, no problem is found. However, when the outer periphery side wears to decrease in height, and develops into a state shown in FIG. 9(b), a top portion 17 of a projected portion 16 which remains in a conical shape on the tip center side of the diamond grindstone body 21 is in contact with a base 23. The base 23 is free of abrasive material such as diamond, rotating quite remarkably slowly at the center, thereby the projected portion 16 is cut inefficiently. Further, the above case is also found where, in place of the projected portion, stones and the like are clogged at the center. Boring is not performed smoothly unless the center portion is cut and, therefore, the bit 20 is once taken out to remove the projected portion 16 and the boring must be started again, which is troublesome. Thus, the boring speed is sharply decreased due to the above reason.    (3) When the boring work is further continued from a state in (2), not only does the boring speed decrease but the diamond grindstone body 21 also lowers in height at the outer periphery portion. At the same time, the outer periphery face of a base 23 (made of iron and free of diamond grains) is more vulnerable to wear than the bit tip, thereby as shown in FIG. 9(c), this part also develops into a tapered shape. The outer periphery face 24 of a bit 20 also acts as a guide portion for providing straight-forward boring. Therefore, if the guide portion 24 is made short after repetition of boring work, there may be found a poor straight-forward boring. When the straight-forward guide is not provided, the bit 20 tends to move in a direction where resistance is small, as indicated by the arrow in FIG. 9(b). Ingredients of concrete are not uniform and the resistance is therefore irregular, by which a hole may be easily deflected. Since, for example, the previously described anchor bolt is formed exactly in a straight shape, it becomes difficult to insert it into the deflected hole. There is also found a decreased capacity in retaining the anchor bolt inside the hole.